The present invention relates to a sputtering target (which may be referred to merely as a target hereinafter), a transparent conductive oxide made of the sputtering target, and a process for producing the sputtering target.
The present invention relates particularly to a target making it possible to suppress nodules generated when sputtering is used to form a transparent conductive oxide into a film, and to make the sputtering stable, a transparent conductive oxide made of such a target, and a process for producing such a target.
In recent years, display devices have been remarkably developed. Liquid crystal display devices (LCD), electroluminescence display devices (EL), field emission displays (FED) or the like are used as display devices for office machines such as personal computers and word processors, or for control systems in factories. These display devices have a sandwich structure wherein a display element is sandwiched between transparent conductive oxides.
As such transparent conductive oxides, indium tin oxide (which may be referred to ITO hereinafter) formed into a film by sputtering, ion plating or vapor deposition accounts for the main current, as disclosed in document 1: xe2x80x9cTechnology on Transparent Conductive Filmxe2x80x9d (published by Ohmsha Ltd., 166th committee on transparent oxides and photoelectric materials in Japan Society for the Promotion of Science, 1999).
Such ITO is made of indium oxide and tin oxide of given amounts and is characterized by having superior transparency and conductivity, being capable of being etched with a strong acid, and having good adhesion to a substrate.
In the meantime, as disclosed in JP-A-3-50148, JP-A-5-155651, JP-A-5-70943, JP-A-6-234565 and so on, there is known a target made of indium oxide, tin oxide and zinc oxide of given amounts, or a transparent electrode made from such a target into a film (which may be referred to as IZO hereinafter). This can be etched with a weak acid, and is widely used because it has good sinterability and transparency.
As described above, ITO or IZO has superior performances as the material of a transparent conductive oxide, but has a problem that when a target is used to form ITO or IZO into a film, nodules (swellings) as shown in FIG. 2 (photograph) are easily generated in the surface of the target.
Particularly when an amorphous ITO film is formed to improve etching property, there arises a problem that the surface of a target is reduced because of a very small amount of water or hydrogen gas introduced into a sputtering chamber for the deposition, so that nodules are more easily generated.
When such nodules are generated in the surface of the target, the nodules are easily scattered by power of plasma during the sputtering. As a result, there arises a problem that the scattered substances adhere, as alien substances, to the transparent conductive oxide during or immediately after the formation of the film.
The nodules generated in the surface of this target account for one of causes of abnormal electric discharge.
Thus, as disclosed in JP-A-8-283934, as measures for suppressing the generation of nodules in a target, efforts are made to reduce pores by making the material of the target denser through sintering at high temperature. That is, a target having a 99%-density relative to the theoretical density thereof is produced, but even in this case nodules cannot be completely removed.
In such a situation, it is desired to develop a target making it possible to suppress the generation of nodules when a film is formed by sputtering and make the sputtering stable.
Thus, the present inventors made eager investigations repeatedly to solve the above-mentioned problems. As a result, the inventors have found out that nodules generated in the surface of a target are basically the remainders of excavation and the cause for generating this remainder of excavation depends on the value (for example, 10 xcexcm or more) of the crystal grain size of metal oxides constituting the target.
Namely, in the case that a target is struck out from the surface thereof by sputtering, the speeds of the striking-out are different dependently on the directions of crystal planes so that unevenness is generated in the surface of the target. It has been proved that the size of the unevenness depends on the crystal grain size of metal oxides present in the sintered body.
Therefore, it can be considered that when a target made of a sintered body having a large crystal grain size is used, unevenness generated in the surface of the target gradually gets larger so that nodules are generated from the convex portions of the unevenness.
That is, an object of the present invention is to provide a target making it possible to suppress nodules generated when a transparent conductive oxide is formed into a film by sputtering, and to make the sputtering stable, a transparent conductive oxide made of such a target, and a process for producing such a target.
[1] According to the present invention, provided is a sputtering target comprising at least indium oxide and zinc oxide, wherein the atomic ratio represented by In/(In+Zn) is set to a value within the range of 0.75 to 0.97, a hexagonal layered compound represented by In2O3(ZnO)m wherein m is an integer of 2 to 20 is contained, and the crystal grain size of the hexagonal layered compound is set to a value of 5 xcexcm or less.
That is, by limiting the value of the crystal grain size into the given range, the size of unevenness generated in the surface of the target is controlled so that the generation of nodules can be effectively suppressed.
[2] When the sputtering target of the present invention is constructed, it is preferred that 67 to 93% by weight of indium oxide, 5 to 25% by weight of tin oxide and 2 to 8% by weight of zinc oxide are contained, and the atomic ratio of tin to zinc is set to a value of 1 or more.
Such a construction makes the target dense and makes it possible to suppress the generation of nodules more effectively.
By setting the atomic ratio of tin to zinc to a value of 1 or more in this way, the electric resistance after crystallization is effectively lowered so that a transparent conductive oxide having superior conductivity can be obtained.
[3] When the sputtering target of the present invention is constructed, it is preferred that a spinel structural compound represented by Zn2SnO4 is contained instead of the hexagonal layered compound or together with the hexagonal layered compound, and the crystal grain size of the spinel structural compound is set to a value of 5 xcexcm or less.
Such a construction makes the target dense so that the generation of nodules can be more effectively suppressed.
Such a construction also makes it possible to yield a transparent conductive oxide having superior transparency and conductivity by sputtering.
[4] When the sputtering target of the present invention is constructed, it is preferred that the bulk resistance thereof is set to a value less than 1xc3x9710xe2x88x923 xcexa9xc2x7cm.
Such a construction makes it possible to reduce abnormal electric discharge (spark) during the sputtering and yield a sputtering film stably. Conversely, if the bulk resistance is a value of 1xc3x9710xe2x88x923 xcexa9xc2x7cm or more, charges are accumulated in the surface of the target so that abnormal electric discharge is easily generated.
[5] When the sputtering target of the present invention is constructed, it is preferred that the density thereof is set to a value of 6.7 g/cm3 or more.
Such a construction makes it possible to give superior mechanical property, and makes the target dense, whereby the generation of nodules can be more effectively suppressed.
[6] Another embodiment of the present invention is a transparent conductive oxide (amorphous transparent conductive oxide) comprising a sputtering target wherein the atomic ratio represented by In/ (In+Zn) is set to a value within the range of 0.75 to 0.97, a hexagonal layered compound represented by In2O3(ZnO)m wherein m is an integer of 2 to 20 is contained, and the crystal grain size of the hexagonal layered compound is set to a value of 5 xcexcm or less.
Such a construction makes it possible to yield an amorphous transparent conductive oxide good in transparency and conductivity effectively.
[7] When the transparent conductive oxide of the present invention is constructed, it is preferred that 67 to 93% by weight of indium oxide, 5 to 25% by weight of tin oxide and 2 to 8% by weight of zinc oxide are contained in the sputtering target, and the atomic ratio of tin to zinc is set to 1 or more.
Such a construction makes it possible to yield an amorphous transparent conductive oxide good in transparency and conductivity effectively. [8] The transparent conductive oxide of the present invention is preferably crystallized at a temperature of 230xc2x0 C. or higher.
Such a construction makes it possible to yield an amorphous transparent conductive oxide better in transparency and conductivity effectively.
Even if the oxide is formed on a substrate, the fear that the substrate is damaged becomes small according to such a temperature.
[9] When the transparent conductive oxide of the present invention is constructed, it is preferred that the half band width of a binding energy peak of the oxygen 1S orbit measured by X-ray photoelectron spectrometer (XPS) is set to a value of 3 eV or less.
Such a construction makes it possible to yield an amorphous transparent conductive oxide good in transparency and conductivity effectively.
[10] When the transparent conductive oxide of the present invention is constructed, it is preferred that the transparent conductive oxide is formed on a substrate or on a colored layer disposed on the substrate.
Such a construction makes it possible to provide a transparent electrode or a transparent electrode having a color filter effectively.
[11] When the transparent conductive oxide of the present invention is constructed, it is preferred that the P-V value thereof according to JIS B0610 is set to a value of 1 xcexcm or less.
If such a construction is used as a transparent electrode, a transparent electrode having a color filter or the like, the generation of wire snapping or short circuits can be effectively prevented.
[12] A further embodiment of the present invention is a process for producing a sputtering target containing a hexagonal layered compound represented by In2O3(ZnO)m wherein m is an integer of 2 to 20, the crystal grain size of the hexagonal layered compound being a value of 5 xcexcm or less, comprising the steps (1) to (3) (the process being referred to as a first production process hereinafter):
(1) blending indium oxide powder with zinc oxide powder having an average grain size of 2 xcexcm or less,
(2) making a formed body wherein the atomic ratio of In/(In+Zn) is within the range of 0.75 to 0.97, and
(3) sintering the formed body at a temperature of 1,400xc2x0 C. or higher.
When the production process is carried out in this way, it is possible to provide effectively a target which makes it possible to suppress the generation of nodules when a transparent conductive oxide is formed into a film by sputtering, and which makes it possible to perform the sputtering stably.
[13] When the first production process of the sputtering target of the present invention is carried out, it is preferred that in the step (1) 67 to 93% by weight of indium oxide, 5 to 25% by weight of tin oxide and 2 to 8% by weight of zinc oxide are blended, and in the step (2) the formed body wherein the atomic ratio of tin to zinc is 1 or more is formed.
When the production process is carried out in this way, it is possible to provide effectively a target which makes it possible to suppress the generation of nodules when a transparent conductive oxide is formed into a film by sputtering, and which makes it possible to perform the sputtering stably.
[14] A still further embodiment of the present invention is a process for producing a sputtering target containing a hexagonal layered compound represented by In2O3(ZnO)m wherein m is an integer of 2 to 20, the crystal grain size of the hexagonal layered compound being a value of 5 xcexcm or less, comprising the steps (1) to (5) (the process being referred to as a second production process hereinafter):
(1) producing a hexagonal layered compound represented by In2O3(ZnO)m wherein m is an integer of 2 to 20,
(2) adjusting the produced hexagonal layered compound to have a grain size of 5 xcexcm or less,
(3) blending the hexagonal layered compound having the adjusted grain size with indium oxide powder,
(4) making a formed body wherein the atomic ratio of In/(In+Zn) is within the range of 0.75 to 0.97, and (5) sintering the formed body at a temperature of 1,400xc2x0 C. or higher.
When the production process is carried out in this way, the hexagonal layered compound whose grain size is beforehand controlled can be used. Therefore, the average grain size in the target can be more easily controlled.
[15] When the first and second production process of the sputtering target of the present invention is carried out, it is preferred that the sintering step is performed in the atmosphere of oxygen gas or pressured oxygen gas.
When the production processes are carried out in this way, it is possible to provide effectively a target which makes it possible to suppress the generation of nodules still more, and which makes it possible to perform the sputtering stably.
[16] When the first and second production processes of the present invention are carried out, it is preferred that the average grain size of the indium oxide powder is set to a value within the range of 0.1 to 2 xcexcm.
When the production processes are carried out in this way, it is possible to provide more effectively a target wherein the crystal grain size of the hexagonal layered compound is controlled into the given range.
[17] When the first and second production process of the present invention are carried out, it is preferred that tin oxide powder is further blended together with the indium oxide powder and the average grain size of the tin oxide powder is set to a value within the range of 0.01 to 1 xcexcm.
When the production processes are carried out in this way, it is possible to provide more effectively a target wherein the crystal grain sizes of the hexagonal layered compound and the spinel structural compound are controlled into the given ranges.
[18] When the first and second production processes of the present invention are carried out, it is preferred that when the formed body is formed, a spinel compound whose grain size is adjusted to 5 xcexcm or less is further blended.
When the production processes are carried out in this way, the spinel structural compound whose grain size is beforehand controlled can be used. Therefore, the average grain size in the target can be more easily controlled.